1. Field of the Invention
The present invention generally relates to an apparatus and method for improving ultrasonic signal generation by an ultrasonic testing device. In particular, the present invention relates to optimizing an ultrasonic response in a manufactured object or part by adaptively adjusting operating characteristics of a laser ultrasound testing device.
2. Description of Prior Art
Ultrasound testing methods are non-invasive, generally non-destructive, techniques used to measure features of materials. These features may include layer thickness, cracks, delamination, voids, disbonds, foreign inclusions, fiber fractions, fiber orientation and porosity. The features influence a given material's qualities and performance in given applications. Each application places unique demands on the material's qualities including the need for differing strength, flexibility, thermal properties, cost, or ultraviolet radiation resistance. With the changing demands, more non-invasive, non-destructive testing of materials is being performed using techniques such as ultrasound testing.
Ultrasound techniques are applied in research as well as industrial settings. In research, ultrasound techniques are used to test new materials for desired features. The techniques are also used to seek defects in material that has undergone stress or environmental endurance testing. In industry, the techniques are used during scheduled servicing to inspect parts for defects. Aircraft, automobile and other commercial industries have shown increasing interest in these techniques.
Ultrasound testing includes transducer-induced, laser, electromagnetic-induced and plasma-initiated ultrasound. Transducer-induced ultrasound techniques use piezoelectric transducers to induce an ultrasonic signal in an object.
Laser ultrasound techniques use a laser pulse. When the laser pulse is directed at an object, it causes thermal expansion in a small region. This thermal expansion causes ultrasonic waves. These ultrasonic waves are then measured by a detector and converted into information about the features of the object. The laser pulse may be generated by several lasers including a ruby laser, a carbon dioxide laser, and a Nd:YAG laser.
In some cases, a higher laser-energy density can be used and some matter at the material surface is ablated. The recoil effect of the pulverized matter launches ultrasonic waves in the material. Similarly to the thermoelastic regime, this ablation regime produces ultrasonic waves can be detected and converted into information about the features of the object.
The electromagnetic-induced ultrasound technique can be used on metallic parts. Coils in the transducer produce an electromagnetic field that induces ultrasonic waves in the sample.
Similar to the laser ultrasound, plasma-induced ultrasound techniques cause thermal expansion initiated ultrasonic waves. Often, a laser generates the plasma by directing the laser at a false target in proximity to the manufactured object. The plasma then hits the manufactured object, causing the thermal expansion and the ultrasonic wave.
The manufactured object may be composed of metal, composite, polymer, ceramic or any other materials. The detector may be one of several devices. For example, the detector may be a transducer on the surface of the object, a laser interferometer directed at the object, or a gas-coupled laser acoustic detector, to name a few.
The measured ultrasonic signals are analyzed to reconstruct physical attributes of the object that have a position or location with the object and a size. A combination of the optical penetration depth in the material given the generation-laser optical wavelength and of the temporal profile or pulse width of the generation-laser pulse dictate the frequency content of the laser-generated ultrasonic waves.
One problem associated with many typical ultrasound detectors is a poor signal-to-noise ratio (SNR) in the resulting signals. SNR is proportional to the amplitude of the sonic wave and inversely proportional to the square root of the bandwidth of the sonic wave. Many typical generation techniques produce low amplitude and/or broad bandwidth ultrasonic signals. These conditions lead to a reduced SNR and limit the quality of data acquired through such ultrasonic testing. In combination, these effects reduce the ability to detect features in the tested object. The reduced SNR in these typical sonic testing systems leads to corresponding reduced resolution in the resulting analysis. Smaller features like fractures may be difficult to detect with the reduced resolution. With the lower SNR of the typical systems, these smaller size features may go unnoticed.
When testing fails to detect features, these features may ultimately yield many problems, such as poor material performance or catastrophic failure of the part. When testing parts used in the airline industry, failure to detect flaws may lead to safety problems and costly accidents.
As such, many ultrasonic testing devices suffer from reduced SNR. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.